Does a Legend Reroll guarantee a Legend fighter?
No. It improves the odds of a high-tier pull but does not guarantee the rarity. Any source telling you otherwise is wrong.
Acquisition
Three reroll types, three sources, and one widely repeated mistake worth correcting.
Updated

Signing happens on a screen headed "Select a fighter or reroll again", which offers a choice of cards plus three separate reroll buttons: Free Reroll, Real Reroll and Legend Reroll.
Free Rerolls are for learning the screen and filling bench slots. Real Rerolls bias the pull toward the orange Real tier. Legend Rerolls bias it toward the red Legend tier.
This is the most repeated error about the game. A Legend Reroll improves your odds; it does not promise the rarity. Plan a Legend hunt around variance, not around a guaranteed outcome.
We are not going to print a percentage. The signing screen does display an odds panel, but the two readings circulating from gameplay videos disagree with each other and neither has been reproduced. Until that panel is captured cleanly, the honest answer is that the odds are not public.
Three sources, and it is worth knowing all of them before you spend:
Because an untrained Legend loses to a trained Real, holding rerolls is usually better than firing them. The training cap means a new card takes real time to become useful, and gym slots cost money.
Redeem both codes, use Free Rerolls to learn the screen, take one fighter through full training blocks until it can clear Bronze, and only then start spending Legend Rerolls — by which point league payouts are topping your reroll points back up anyway.
FAQ
No. It improves the odds of a high-tier pull but does not guarantee the rarity. Any source telling you otherwise is wrong.
Codes give six to start. After that, completing league matches pays Legend Reroll Points — +1 per Bronze match, up to +5 in the third league.
No. An untrained Legend loses to a trained Real, and every new card needs training blocks and a gym slot. Hold them until your gym can actually finish the fighter you pull.